This invention relates to an apparatus for sychronously switching electrical circuits, and to a piezoelectric relay which is especially useful in such an apparatus. More particularly, it relates to piezoelectric circuit elements which are specially constructed to switch load current circuits synchronously in time with so-called zero current time points.
Electromagnetic relays have been used in the past to switch a wide range of electrical circuits by separating or closing one or more pairs of electrical contacts. While electromagnetic relays perform satisfactorily for some applications, they can be slow acting, relatively large in size, and costly. They typically require a relatively bulky solenoid coil and associated linkage to provide contact movement. Such coil and linkage systems are, in addition to being a major part of the relay cost, generally energy inefficient. Furthermore, electromagnetic relays do not lend themselves to synchronous operation. Although conventional electromagnetic relays may be employed to switch such loads as, for example, an alternating current electrical circuit on demand, the movement of the contacts is usually random on the time scale of the load current waveform, because of generally long mechanical reaction times associated with the operation of such relays. As a result, opening and closing operations of the contacts are not synchronized with the zero current points of the current waveforms, especially when the load is an alternating current circuit. For electrical circuits operating at current levels typical of such power sources as household electrical wiring, opening and closing of the relay contacts is often accompanied by arcing between the contacts. When the current in such a circuit is interrupted, the current through the relay contacts does not drop to zero at the instant of contact separation, but rather persists in the form of an arc between the contacts, usually until the alternating current waveform approaches the next sinusoidal zero. As the current level decreases toward the sinusoidal zero point, the arc becomes unstable and suddenly is extinguished, a phenomenon often referred to as chopping. This sudden extinction at low current represents an extremely high rate of change of current. As a result, if the electrical circuit in which the current is being interrupted has significant inductance, high voltage transients, proportional to the product of the inductance and the rate of change of the current, are produced. These voltage transients may cause electrical breakdown in either the equipment connected with the circuit, or the relay itself, or both. Moreover, such arcing is damaging to the contacts themselves and can cause contact erosion and contact welding. It is therefore desirable to minimize any arcing occurring between the relay contacts when the contacts are opened or closed. One way to minimize such arcing is to operate the relay so as to switch the electrical circuit at a point in the load current waveform where the current level is as close to zero as possible, which operation is referred to hereinafter as synchronous operation.
A piezoelectric device, utilizing the fast action capability of a piezoelectric bender, is employed in the present invention to provide a synchronously operable switching relay. Synchronous operation requires that the relay contacts be moved between the open and closed positions in a relatively short period of time. The fast action, relatively low mass, and small travel distance of a piezoelectric bender facilitate the use of such a device in a synchronously operable relay. A further characteristic of a piezoelectric device is that the deflecting force acting to move the contacts is at a maximum at the beginning of the piezoelectric bender's deflection. This characteristic further enhances the device's capability of moving the relay contacts in a short period of time. With this fast action capability, a piezoelectric relay may be operated so that the contacts are opened or closed at a time very close to the time when the current level is zero in the circuit being switched, thereby substantially reducing contact erosion, contact welding, and transient inductive voltages. Also, the simplicity of a piezoelectric device avoids most of the mechanical problems of conventional electromagnetic relays, and the energy efficiency of such a device permits operation with far less expenditure of energy.
Piezoelectric benders have been used in the past in a number of applications, including utilization in various piezoelectric relays. For example, piezoelectric benders used as relay elements are described in U.S. Pat. Nos. 2,166,763, 2,182,340, 2,471,967, 2,835,761, 4,093,883, and 4,403,166. However, none of the piezoelectric relays disclosed by these patents have been specifically designed to minimize arcing. No consideration has been given to providing a synchronously operable relay, or to one which is especially useful for switching electrical circuits operating at household power line current levels. As has been noted above, switching circuits operating at such current levels results in significant arcing if the circuit is not switched at a point in time close to a sinusoidal zero of the alternating current level. Application Ser. No. 684,882, assigned to the same assignee as the present invention and filed concurrently herewith, discloses a synchronously operable electrical current switching apparatus which employs a plurality of piezoelectric benders and which may be used to switch multiple circuits or to lower the contact resistance in one or a few circuits. Application Ser. No. 684,880, also assigned to the present assignee and filed concurrently herewith, discloses a synchronously operable electrical current switching apparatus which provides increased separation between the switching contacts when the contacts are in the open position and increased closing force when the contacts are in the closed position. The present invention provides a piezoelectric relay having a very small gap length as compared to the contact separation for conventional relays, which relay may be synchronously operated so that 110 volt alternating current circuits are switched on and off with minimal arcing between the relay contacts.
Furthermore, while the small deflections characteristic of a piezoelectric bender provide a piezoelectric relay with fast action capability, this same feature limits the amount of voltage that may be applied between the relay contacts before electrical breakdown occurs. If the piezoelectric relay possesses relatively small physical dimensions, so as to fit into a 16 pin dual-in-line integrated circuit package (DIP), for example, the separation between the relay contacts is very small, being much smaller than the contact separation for conventional relays designed for operation at household power line current levels. The present inventor has found that even for very short distances between the contacts, the dielectric strength of a piezoelectric device is sufficient for operation with typical 110 volt household power lines. Unexpectedly, sufficient breakdown voltage for such devices can be achieved even for contact separations as small as 0.1 mil. By Paschen's law the breakdown voltage between two electrodes in a gaseous atmosphere is a function of the product of gas pressure and the distance between the electrodes. For an electrode separation of 1 cm. in air at atmospheric pressure, the breakdown voltage can be determined to be approximately 30 kv. If this breakdown field of 30 kv/cm is used to estimate the breakdown voltage of a 1 mil electrode separation in air, as is permissible for separations of a few centimeters, the estimated breakdown voltage is 77 volts. However, it has been found that the breakdown voltage given by Paschen's law does not linearly decrease to zero as the contact separation becomes small, but rather approaches a minimum value and then begins to increase again. For air, this minimum breakdown voltage has been found to be somewhat greater than 300 volts. It is theorized that the reason for this minimum breakdown voltage and subsequent increase in dielectric strength is that, when the product of gas pressure and electrode separation is small, the number of gas atoms with which an electron can collide in traversing the gap between the contacts also becomes small. Since the breakdown process in a gas causes the gas to become an electrical conductor and therefore depends critically upon the ability of electrons in the gap to collide with and ionize ambient gas atoms, the probability of establishing a conducting path is reduced when the number of available target atoms is small. It is believed that the breakdown voltage approaches a minimum and then increases again because the breakdown process undergoes a fundamental change from a gas collision mechanism to an electrode-surface dominated, vacuum-breakdown mechanism, and also because the breakdown voltage of a given contact separation is generally much higher in vacuum than in a gas at atmospheric pressure. Accordingly, the present inventor has determined that a piezoelectric relay having a very small separation between the contacts may be synchronously operated so that 110 volt alternating current circuits are switched on and off with minimal arcing between the relay contacts.
It is seen from the above that it is an object of the present invention to provide a synchronously operable electrical current switching apparatus that functions with minimal arcing between the switch contacts.
It is a further object of the present invention to provide an electrical current switching apparatus which is fast acting, small in size, highly energy efficient, and low in cost.
It is another object of the present invention to provide a rapidly operable piezoelectric relay including a piezoelectric bender which is especially useful in synchronously operating electrical circuits.
It is also an object of the present invention to provide a compact piezoelectric relay which may be enclosed in a 16 pin dual-in-line integrated circuit package.